With more and more new folks coming to Linux via *ubuntu, some have had trouble getting this to work on the newest version of their distro. For those without the libglew2.0 package, which is not stored on the newest repositories, you can safely install it by adding deb http://cz.archive.ubuntu.com/ubuntu bionic main universe to the end of the sources.list file at /etc/apt/sources.list. Then run sudo apt update && sudo apt install libglew2.0.
Afterwards, I would recommend commenting out the new line you added in sources.list by putting a # at the beginning of the line. You can also just download libglew2.0 directly from the internet, but that's somewhat poor form (and not as secure) as grabbing it from the repository via command line.
On a fresh(-ish) 20.04, you should already have libsdl2-2.0-0, but probably won't have libsdl2-mixer-2.0-0.
As a final note, you need not worry about libglew2.1 in focal fossa's repositories overwriting 2.0.
